In E-UTRAN, OFDMA technology is used for downlink transmission, see 3GPP TR 25.814, “Physical Layer Aspects for Evolved UTRA”. OFDM waveforms have typically high peak to average power ratio (PAPR), see 3GPP TR 25.814, “Physical Layer Aspects for Evolved UTRA” and R4-060853, Ericsson, Reserved sub-carriers for LTE. High PAPR leads to inefficient radio implementation since radio frequency (RF) components such power amplifiers have to be over dimensioned to account for peak (or maximum) transmission power levels, which occurs infrequently. Therefore, techniques are needed to reduce the high Peak to Average Power Ratio (PAPR).
In the following, various technological aspects, requirements, and limitations related to high bit rate transmission in E-UTRAN systems are described.
In E-UTRAN very high downlink data rate is envisaged. In order to achieve the high data rate, techniques such as higher order modulation and MIMO schemes will be used. High order modulation such as 64 QAM or 16 QAM used in conjunction with MIMO could provide very high data rate provided the modulation quality levels of the signal at the transmitter and at the receiver are sufficiently good. High modulation quality would imply high SINR, which is required to achieve a high data rate since it would allow higher order modulation to be used. Error vector magnitude (EVM) is one of well known and reliable performance metric to judge the modulation quality. EVM is the measure of the level of impairment of the transmitted and received signal. It is also used in UTRAN for both base station and UE transmitter modulation quality (see TS 25.104, Base Station (BS) radio transmission and reception (FDD), and TS 25.101, User equipment (UE) radio transmission and reception (FDD)). The same measure (EVM) will also be used in E-UTRAN to specify the modulation quality.
It has been shown that the downlink transmitter EVM requirement for 64 QAM in E-UTRAN is in the order of 4% in order to minimize significant throughput loss (see R4-061172, Ericsson, Reserved sub-carriers for LTE). The results on throughput loss for different EVM levels are shown in FIG. 1.
An OFDMA waveform itself has inherently high PAPR. OFDMA waveform coupled with higher order modulation to achieve high data rate also leads to even worse PAPR, i.e. very high PAPR. High PAPR is not desirable from radio implementation point of view. Therefore, techniques are used to reduce PAPR but such techniques should not degrade the EVM requirement needed for high data rate. In order to reduce the PAPR the signal is generally clipped at the transmitter. The clipping is a non linear operation; on the one hand it reduces the PAPR, but on the other hand it would also introduce additional noise, which is often referred to as clipping noise. Therefore, a technique is needed to eliminate the clipping noise or at least reduce its effect on the useful sub-carriers that carry signaling or data.
A well known technique is “Tone Reservation” or sub-carrier reservation. This scheme requires a small percentage of reserved sub-carriers (sub-carriers not carrying data) dedicated to PAPR reduction. The clipping noise is incorporated by the transmitter into the reserved sub-carriers, which are eventually discarded by the UE as explained further below. The needed percentage of reserved sub-carriers depends on various factors such as the FFT (Fast Fourier Transform) size, goal for PAPR, number of iterations for the algorithms to converge, etc, but is typically small. By introducing some reserved sub-carriers, PAPR is reduced while maintaining low EVM (high modulation quality) on the data symbol. This consequently preserves the needed SNR for higher order modulations and coding schemes to reach high peak rates.
As stated above the clipping noise from pilot or data sub-carriers are added to the reserved sub-carriers. This makes the pilot or reference symbols less noisy. The channel estimation and eventually the demodulation at the UE are done on the reference or pilot symbols. Thus another advantage of the reserved sub-carriers is that it improves the demodulation at the UE.
Considering a PAPR reduction scheme utilizing reserved sub-carriers with target PAPR of 7 dB, 16 iterations and IFFT (Inverse Fast Fourier Transform) size of 512, the percentage of the reserved sub-carriers versus data symbol EVM can be obtained from the curve in FIG. 2.
Given the proposed transmitter EVM of 4%, we would need to limit the residual EVM due to PAPR reduction to 1.5%-2% which indicates that 5% reserved sub-carriers would be sufficient.
Reserved sub-carriers imply a spectrum efficiency loss of 5% but the degradation in throughput by not having sufficient EVM requirements would be significantly higher then a 5% spectrum efficiency loss due to reserved sub-carriers (see FIG. 1).
The reserved sub-carriers are generally spread over the entire cell transmission bandwidth with a certain pattern. The pattern may depend upon the cell bandwidth size. However, for a given bandwidth and number of reserved tones the pattern is generally the same.
In E-UTRAN the broadcast channel (BCH) is transmitted in the center of cell transmission bandwidth. The bandwidth of BCH is 1.25 MHz. Furthermore, the BCH is not transmitted continuously in time. The current working assumption is that it is transmitted once or twice (one or two TTI) per 10 ms frame.
The downlink L1/L2 control information in E-UTRAN is used for resource allocation. In a cell there are more than one L1/L2 control channels. A downlink L1/L2 control channel can be sent to a single UE or to a group of UE in a cell.
As for Peak Reduction in WCDMA, the downlink waveform without peak reduction schemes has high PAPR (e.g. ˜10-12 dB) in UTRA (see TS 25.141, Base Station (BS) conformance testing (FDD)). There are efficient schemes based on implementation such as clipping to reduce the PAPR without violating the modulation quality requirements (EVM and PCDE, Peak Code Domain Error) stated for both QPSK and 16 QAM modulations in TS 25.104, Base Station (BS) radio transmission and reception (FDD). Hence, in UTRA no reserved tones or symbols are used to reduce PAPR.
The concept of reserved sub-carriers to reduce PAPR is a well known concept. It should be noted that the terms tones and sub-carriers are interchangeably used in the literature but they have the same meaning. However, the general assumption is that they are always transmitted in a cell. This leads to wastage of bandwidth when reserved tones (or sub-carriers) are not needed to achieve target PARP, e.g. in low-data rate scenarios. Secondly, the pattern of the reserved sub-carriers needs to be standardized in order to ensure that UE receiver knows the occurrence and pattern of the reserved sub-carriers. This approach does not provide any flexibility to the network since the same pattern is to be used. The pattern is generally characterized by the positions or frequency of occurrence of sub-carriers within the cell transmission bandwidth in the frequency domain.